1. Field of the Invention
The present invention relates to a wavelength division multiplexed optical transmission system which transmits a wavelength division multiplexed optical signal using a dispersion-shifted fiber.
2. Background Art
Wavelength Division Multiplexing (WDM) transmission technology is a technology in which optical signals of differing wavelength (optical frequency) are multiplexed, and transmitted via one optical fiber transmission path. Here, the optical signal is the optical output of a light source directly modulated by a data signal (direct modulation type), or an optical transmission wave output from a light source modulated by a data signal using an external modulator (external modulation type), and this wavelength is determined by the light source wavelength.
By disposing along the optical fiber propagation path optical amplifiers which amplify the optical signal as-is, and compensating the transmission loss of the optical fiber transmission path, it is possible to extend the span between regenerative repeaters which are necessary for discriminative reproduction processing at the electrical step. This optical amplifier can increase the transmission capacity of an installed optical fiber transmission path by many times the number of wavelengths simply by altering the transmission and receiving apparatuses for wavelength division multiplexing use because it possesses a function in which optical signals of differing wavelength are amplified together. For example, the amplification wavelength bandwidth of an erbium doped optical fiber amplifier (EDFA) is between 1.53 μm and 1.56 μm, and by multiplexing optical signals at wavelength intervals of 0.8 nm in this wavelength band, about 30 channels of optical signals can be transmitted through in one optical fiber.
However, installed dispersion-shifted fibers transmit optical signals of a designed zero-dispersion wavelength. When transmitting wavelength division multiplexed optical signals in this dispersion-shifted fiber, cross-talk due to four-wave mixing, a non-linear optical effect, is generated, and because of this the input power to the transmission path fiber could not be increased. In the following this problem will be explained in detail.
The propagation loss of a silica optical fiber is minimal in the 1.5 μm to 1.6 μm region. A dispersion-shifted fiber is designed so that the wavelength dispersion is zero in the 1.55 μm wavelength region, and by suppressing waveform degradation due to wavelength dispersion at this wavelength, the transmission distance can be increased. In addition, while the International Standards Organization has stipulated that the zero dispersion wavelength of a dispersion-shifted fiber is allocated between 1.525 μm and 1.575 μm, practically the distribution is roughly between 1.535 μm and 1.565 μm, centered on 1.550 μm, and up to the present, these have been widely installed.
In contrast, when optical signals of differing optical frequencies are input into an optical fiber, new optical frequencies dependent on the difference in input optical frequencies are generated based on third-order non-linearity within the optical fiber. This is called “four-wave mixing,” and is a phenomenon wherein, for example, an optical frequency f1+f2−f3 is generated from input optical frequencies f1, f2, and f3. This four-wave mixing is more easily generated the smaller the dispersion value of the input optical wavelength, or the larger the input power of each individual wavelength.
If the optical frequency intervals between the wavelength division multiplexed optical signals input into this kind of optical fiber are uniform, the optical frequency newly produced by four-wave mixing will conform with one optical wavelength among those of the optical signal, and strong noise will be generated by mutual interference. In addition, even when the optical frequency intervals of the wavelength division multiplexed optical signal are not uniform, the optical power of the original optical signal is consumed in the generation of four-wave mixing, and this produces strong noise. When the optical frequency interval of the wavelength division multiplexed optical signal has even spacing, excess noise originating in four-wave mixing is generated by an input power per wavelength from about −5 dBm, and when the spacing is uneven, it is generated by an input power per wavelength from about −2 dBm. Because of this, the optical power that can be input into the optical fiber transmission path cannot exceed this value, and as a result, the transmission distance is limited.